A light emitting diode (LED) lighting system requires a power converter for changing the AC line voltage received from the utility power line to the DC power specific to the LED load circuit. A simple system for this purpose produces output current to the LED load whenever the incoming AC line voltage is sufficiently removed from zero. The power converter input current can be designed to represent a resistive load to the AC line input, giving a high power factor for the AC line input. In addition, the power converter input current may be sufficient to cause a phase cut lamp dimmer with either leading edge (LE) or trailing edge (TE) phase cut to operate correctly.
One difficulty with this simple means of operation is that the majority of LE and TE phase cut dimmers have a limited control range. For example, if the AC half cycle is denoted to have 180 degrees of electrical phase shift in its half period, the dimmer may typically produce an output with a minimum of 40 degrees phase delay cut, and a maximum of 140 degrees phase delay cut. The half period always has the first 40 degrees of the undimmed AC voltage missing, and always has AC power for the last 40 degrees of AC input voltage. As a result, when the dimmer is set to the maximum phase delay to give the minimum light output, there is still substantial AC power being converted to load power. Practical dimmers never have exactly the angle range or limits stated above, so these values given are exemplary of typical performance The minimum phase delay is usually in the range of 30 to 50 degrees, and the maximum phase delay is usually from 130 to 150 degrees, with some going all the way to 180 degrees.